Plasma accelerator system

ABSTRACT

A multistage plasma accelerator system includes at least one intermediate electrode between the plasma chamber between electrodes that include each other. An especially good efficiency can be achieved by way of an uneven distribution of potential to the potential stages formed by the plurality of electrodes having a high potential gradient of the last stage, when the plasma beam emerges, and by a special shape of the magnetic field prevailing in the plasma chamber of the last stage.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of German Application No.101 53 723.9 filed Oct. 31, 2001. Applicants also claim priority under35 U.S.C. §365 of PCT/EP02/12095 filed Oct. 30, 2002. The internationalapplication under PCT article 21(2) was not published in English. Theinvention relates to a plasma accelerator system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Plasma accelerator systems serve, for example, as drives for spacemissiles. In this connection, a working gas is ionized in a plasmachamber, and the ions are accelerated in an electrostatic field andexpelled as a neutralized plasma beam, by means of electrons that aresupplied.

2. The Prior Art

The most common embodiment type of such plasma accelerator systems isthe so-called Hall thruster, whose ring-shaped plasma chamber has anessentially radial static magnet passing through it. Such Hall thrustersare known, for example, from EP 0541309 A1 or U.S. Pat. No. 5,847,493.

In the case of these Hall thrusters, an electron source arranged outsideof the plasma chamber, on the side of its beam exit, and laterallyoffset relative to the latter, emits an electron stream that is partlypassed into the plasma chamber, under the influence of the electricfield between the electron source and an anode arranged at the bottom ofthe plasma chamber, as ionization electrons, and partly carried along bythe ions that exit from the chamber, as neutralization electrons. Theionization electrons are deflected in the plasma chamber, under theinfluence of the magnetic field, and form ring-shaped drift streams,whereby the duration time and the ionization effect on the working gasthat is introduced into the plasma chamber is significantly increased.

DE-AS 1222589 shows a plasma accelerator system, in which an arcdischarge is ignited in a plasma chamber delimited in length by an anodeand a cathode. The resulting ions are drawn off by a ring-shaped ionacceleration electrode arranged outside of the plasma chamber andseparated from the latter by an insulated electrode, and expelled inaccelerated manner. An energy-rich bundled electron beam supplied fromthe cathode side, on the center axis of the system, runs through theplasma chamber and exits through the acceleration electrode with theelectrons of the electron beam, and neutralizes the ion beam. Theelectrons that are formed during the arc discharge and the electrons ofthe supplied beam that are braked by means of pulse processes perform anoscillating movement between the ion acceleration electrode and thecathode. A magnetic collimator field that runs parallel to thelongitudinal axis bundles the particle streams about the center axis.Additional electrostatic acceleration stages having magnetic bundlingcan follow the acceleration electrode.

A plasma accelerator is described in Patent Abstracts of Japan 09223474,which has a plasma generator chamber and a plasma accelerator chamber,one after the other, through which working gas is passed, in eachinstance. A coil arrangement generates a beam-parallel magnetic field.Several stabilization electrodes that surround the beam are arranged inthe two chambers, one after the other.

A plasma accelerator system is known from DE 19828704 A1, in which anenergy-rich bundled electron beam is introduced into a plasma chamberdelimited, in the longitudinal direction, by an anode and an endelectrode, and passed through a magnet arrangement along the centeraxis. Several intermediate electrodes are provided in the longitudinaldirection between the anode and the end electrode, which divide thepotential difference between the anode and the end electrode intoseveral stages. The magnet arrangement shows the particular feature thatthe magnetic field it generates in the plasma chamber periodicallychanges polarity in the longitudinal direction, and that alternatingfield segments of the first type and the second type occur in thelongitudinal direction, whereby in the segments of the first type, thefield lines run predominantly radially, i.e. perpendicular to thelongitudinal direction, and in the segments of the second type, thefield lines run predominantly axially, i.e. parallel to the longitudinaldirection. The segments of the first type preferably lie between twoconsecutive electrodes, in the longitudinal direction, and form barriersfor the electrons accelerated towards the anode. A system structured inthis way, in several stages, having the electron barriers, makes itpossible to increase the degree of effectiveness of the plasmaaccelerator. DE 10014033 A1 describes a plasma accelerator system havinga similar magnetic field arrangement for a ring-shaped plasma chamberand an electron source that lies on the outside, at the end of theplasma chamber. A plasma accelerator system known from DE 10014033 A1provides for introduction of the electrons accelerated from the anodeside, into a ring-shaped plasma chamber, in the form of a cylindricalhollow beam, into the plasma chamber.

U.S. Pat. No. 6,215,124 B1 describes an ion accelerator according to thetype of a Hall thruster, having a ring-shaped plasma chamber and anessentially radial magnetic field between a first magnetic pole thatlies radially on the inside and a second magnetic pole that liesradially on the outside. As a particular feature, it is provided herethat several electrodes are arranged, in electrically insulated manner,at different radial distances from the exit of the plasma chamber, onthe beam exit side of the plasma chamber, at its face that points in thebeam direction, which lies essentially crosswise to the beam directionand outside of the plasma chamber, which electrodes lie at differentintermediate potentials between the cathode potential and the anodepotential, or even below them. A maximum of the longitudinal gradient ofthe magnetic field is shifted in the direction of the exit of the plasmachamber, and preferably outside it, by means of a magnetic short-circuitabout the anode region. A field lens that counteracts the divergence ofthe ion beam can be generated in the electrostatic acceleration field,by means of the intermediate electrodes on the outside face, and themaximum of the acceleration field can be moved behind the beam exitopening, in the beam direction.

SUMMARY OF THE INVENTION

The present invention is based on the task of further improving such aplasma accelerator system, particularly with regard to the degree ofeffectiveness.

The invention is described in claim 1. The dependent claims containadvantageous embodiments and further developments of the invention.

By means of the gradation, according to the invention, of the potentialdifference that exists over the length of the plasma chamber, into afinal potential stage on the exit side, having a relatively highpotential difference, and one or more potential stages on the anodeside, having a comparatively smaller potential difference, in the ratiosindicated more precisely in the claims and below, a great potentialdifference is available in the acceleration stage and therefore at alocation where the ion concentration is already high because of theionization of the preceding stages, for acceleration of the ions to agreat velocity, and therefore a great impulse, whereas the potentialdifference of the preceding stages, which is less, in comparison, isparticularly advantageous for the ionization of the working gas. At thesame time, however, the acceleration stage is also available forreproduction of the ionization electrons supplied there, by means ofpulse ionization, and the resulting secondary electrons.

In this connection and in the following, ionization electrons areunderstood to mean the electrons that are accelerated towards the anodein the electrostatic field, and generate the positively charged ions ofthe working gas by means of their movement that is influenced by themagnetic field. At the same time, the term ionization electronsdistinguishes these electrons from the electrons designated asneutralization electrons, which are given off to the outside togetherwith the accelerated ion beam and guarantee a charge-neutral plasmabeam. Ionization electrons and neutralization electrons can come fromthe same electron source, at least in part.

The segment between an end electrode arranged at the exit of the plasmabeam from the plasma chamber, and an intermediate electrode that comesnext to it, in the direction towards the anode, is referred to as a lastor exit-side potential stage. The potential difference between the endelectrode and the next intermediate electrode that occurs in thispotential stage is referred to as the last potential difference.

The magnetic field configuration that is present in the plasma chamber,in connection with the electrode arrangement within the plasma chamber,preferably in the form of a sequence of segments of the first type,having field lines that run predominantly radially, i.e. perpendicularto the longitudinal direction of the plasma chamber, alternating withsegments of the second type, having field lines that run predominantlyaxially, i.e. parallel to the longitudinal direction of the plasmachamber, is of particular significance for the invention, as is, inparticular, the magnetic field that exists in the plasma chamber, withthe magnetic field segment in the last potential stage, in combinationwith the great potential difference of the exit-side last potentialstage. The intermediate electrodes preferably lie between adjacentmagnetic field segments of the first type, having a predominantly radialprogression of the magnetic field.

In the last potential stage, in particular, a magnetic field segment ofthe first type prevents ionization electrons passed to the lastpotential stage from being highly accelerated, and impacting one of thenext electrodes, with the loss of the absorbed energy. Rather, amagnetic field segment of the first type forms a barrier for theelectrons accelerated in the electrostatic field, in that the latter areforced onto drift paths having a movement component that runspredominantly crosswise to the longitudinal direction, and reduce theenergy from the electrostatic field, step by step, by means of pulseionization, until they overcome the barrier. In this connection, a highreproduction factor of the ionization electrons is already obtained,even in the next potential stage, referred to as the last stage, so thatthe last potential stage already passes a high number of electrons on tothe next-to-last potential stage.

In this connection, it is advantageous if the magnetic field segment ofthe first type in the last potential stage lies between the electrodesthat form the last stage, particularly in a region where theelectrostatic field runs essentially axially and has high values. Theions are not influenced in their movement by the magnetic field, to anynoteworthy extent, and are highly accelerated axially by means of theelectrostatic field of the last potential stage, whereby it isadvantageous that because of the great non-uniformity of the potentialstages, according to the invention, the high acceleration in thelongitudinal progression of the plasma chamber does not set in until theregion in which the degree of ionization of the working gas is veryhigh, so that the last potential stage, which comprises almost theentire potential difference of the system, can be essentially utilizedfor the acceleration of all working gases.

It is advantageous if the last potential difference amounts to at leastfour times, particularly at least ten times the first potentialdifference, i.e. the potential difference between the electrode thatfaces away from the plasma exit and the next intermediate electrode inthe direction of the plasma exit. The segment between the anode and theintermediate electrode next closest to it is referred to as the firstpotential difference.

In the case of more than one intermediate electrode between the anodeand the end electrode, additional intermediate potential stages betweenconsecutive intermediate electrodes occur accordingly. It is thenadvantageous if the potential difference of the last potential stageamounts to at least four times, particularly at least ten times thegreatest potential difference of the other potential stages.

It is advantageous if the last potential difference is greater than thesum of the other potential differences, and if it amounts to preferablyat least two times, particularly at least four times the sum of theother potential differences.

It proves to be advantageous that the intermediate potentials of theintermediate electrodes do not have to be predetermined in fixed andcompulsory manner, but rather that one or more intermediate electrodescan also lie at sliding potentials.

According to an advantageous embodiment, the end electrode can be formedby an electrode that surrounds the plasma chamber at the exit of theplasma beam and/or delimits it laterally. In another advantageousembodiment, the end electrode can also be arranged outside the plasmachamber, at the plasma beam exit, particularly also according to thetype of the cathodes of the Hall thruster systems, with a lateraloffset.

The ionization electrons that initiate ionization can be passed to thelast potential stage in known manner. For example, an acceleratedelectron beam can be introduced into the plasma chamber from the anodeside of the latter, and be centrally guided in the longitudinaldirection by means of the magnetic field arrangement. The electrons ofthe electron beam ES are braked in the electric field. One part of theelectrons of the electron beam is deflected at the end of the firstpotential stage, and accelerated towards the anode as ionizationelectrons. Another part of the electrons of the electron beam exits fromthe chamber with the working gas ions, as an electrically neutral plasmabeam. In another manner, similar to the Hall thrusters, an electronsource is arranged outside the plasma chamber, near the exit of theplasma beam, with a lateral offset, and emits an electron stream that ispartly passed into the plasma chamber as ionization electrons, throughthe plasma beam exit, and partly carried along by means of the volumecharge effects of a non-neutralized ion beam, and causes an electricallyneutral plasma beam to be issued. In yet another embodiment, anelectrode can be provided at the exit of the plasma beam from the plasmachamber, which electrode is exposed to a border region of the plasmabeam. The ions, which are already highly accelerated at this position,release an electron shower upon impact on this electrode and/or releaseelectrons due to volume charge effects, which again are partlyaccelerated as ionization electrons, in the anode direction, and partlycarried along to neutralize the plasma beam. To generate an initial ionstream, a gas discharge can be ignited by means of briefly raising thegas pressure and/or the potential difference of the last potentialstage, for example. However, a start can take place solely by means ofspontaneous ionization, e.g. by means of high-energy cosmic radiation.The different types of electron sources can also be implemented incombined manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail below, using preferredexemplary embodiments, making reference to the figures. These show:

FIG. 1 a longitudinal cross-section through a plasma chamber,

FIG. 2 a system having an electron source located on the outside,

FIG. 3 a system having an ion-impacted electrode as the electron source.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the plasma accelerator system shown in FIG. 1, a plasma chamber PK isstructured essentially as a circular cylinder about the longitudinalaxis LA. The plasma chamber is surrounded by several electrodes EA, EZ1,EZ2, EE, preferably ring-shaped, that follow one another at a distancein the longitudinal direction LR and are at different potentials. Aworking gas AG, preferably xenon, is passed to the plasma chamber.

A tightly bundled, highly accelerated electron beam ES from a beamsource, not shown, is introduced into the plasma chamber on thelongitudinal axis, from the side of the first electrode EA, alsoreferred to as an anode, and centrally passed through the magnetic fieldMF of a magnet arrangement that surrounds the plasma chamber, on thelongitudinal axis LA.

The potential progression over the different potentials of the separateelectrodes is monotonous in the longitudinal direction LR and directedin such a manner that the electrons of the electron beam are brakedalong their path through the plasma chamber, and positively charged ionsof the working gas, generated in the plasma chamber, are accelerated inthe direction of the electrode EE, which is arranged as the lastelectrode of the series, at the beam exit SA of the plasma chamber. Ionsand electrons NE leave the plasma chamber at the beam exit, as anelectrically neutral plasma beam PB.

The magnet arrangement is schematically represented by several magnetrings MR that surround the plasma chamber, which alternately haveopposite poling, following one another in the longitudinal direction.

Such a magnet arrangement produces a magnetic field in the plasmachamber, which has segments MA1A, MA1Z, MA1E of the first type, in thelongitudinal direction, at positions between consecutive magnet rings,in which the magnetic field MF is predominantly directed radially.

The magnetic field segments of the first type form electron barriers inthe potential stages formed by two consecutive electrodes, in eachinstance, having a first potential difference PDA for the first,anode-side potential stage between the anode EA and the firstintermediate electrode EZ1, an intermediate potential difference PDZ foran intermediate stage between the first (EZ1) and the second (EZ2)intermediate electrode, and a last, exit-side potential difference PDFfor the last potential stage between the second intermediate electrodeEZ2 and the end electrode EE, in that electrons accelerated in theelectrostatic field EF of the electrode arrangement, at a distance fromthe longitudinal axis, are deflected by the magnetic field and held in astage for a long time. As a result, the probability of the ionizinginteraction with the working gas and therefore also the measure ofreproduction of the electrons by means of the secondary electronsreleased during ionization is greatly increased.

According to the invention, the potential difference PDE of the lastpotential stage amounts to at least four times, particularly at leastten times the potential difference PDA of the first potential stage or,in the case of more than two potential stages, to at least four times,particularly at least ten times the greatest of the potentialdifferences PDA, PDZ of the other potential stages. It is advantageousif these potential differences PDA, PDZ of the other potential stagesare less than the last potential difference PDE, and preferably amountto a maximum of 50%, particularly a maximum of 25%, of the lastpotential difference PDE. For example, a selection can be made so thatPDA=50 V, PDZ=50 V, and PDE=900 V.

The number of electrons suitable for ionization increases steeply fromstage to stage, from the last potential stage to the first potentialstage, as a result of the reproduction factor. The major portion of theionization of the working gas therefore lies in the potential stages PDAand PDZ. Because of the magnetic field segment MA1E of the first type inthe last potential stage, however, electron beams that are greatlybraked in this stage, in the electron beam that is introduced, are heldin this stage for a long time and thereby already generate a largenumber of secondary electrons, which are transferred to the next stagein the direction towards the anode. At the same time, the concentrationof the ions accelerated in the direction from the anode EA to the endelectrode EE has approximately reached its maximum upon entry into thelast potential stage, so that the great potential difference of thislast potential stage is essentially available as an accelerationpotential for the entire ion stream.

The combination of the high last potential difference PDE and themagnetic field segment MA1E in the last potential stage therefore leadsto a particularly good degree of effectiveness of the plasma acceleratorsystem.

It is advantageous if the other potential stages also have magneticfield segments MA1A, MA1Z of the first type, which alternate withmagnetic field segments MA2 of the second type, following one another inthe longitudinal direction, in which the magnetic field in the plasmachamber runs predominantly axially, i.e. parallel to the longitudinaldirection. A particularly high ionization portion is achieved in thefirst potential stage.

For a better differentiation, magnetic field segments of the first andthe second type are shown spaced apart by transition segments in thefigures.

Because of the progression of the magnetic field, divergent from thelongitudinal axis, in the segments of the first type and thepredominantly axial progression in the segments of the second type, theelectrons are kept away from the lateral electrodes, for the most part,and are maintained as ionization electrons.

While the initial ionization electrons IE are obtained in the lastpotential stage in that part of the electrons of the electron beam thatis introduced does not overcome the potential of the end electrode andis branched out of the electron beam and accelerated in the oppositedirection, in the system shown in FIG. 1, an embodiment shown in FIG. 2for the region of the plasma beam exit SA provides a cathode arrangedoutside the plasma chamber PK1, in the manner of a Hall thruster, as theelectron source QE, the emitted electron stream of which is passed inpart to the plasma chamber, as ionization electrons IE, through the beamexit SA, and in part carried along by the plasma beam PB, asneutralization electrons NE. In the case of such an arrangement, the endelectrode can be formed by this cathode, so that the last potentialstage is formed between the cathode EQ and the intermediate electrodeclosest to the exit.

In the plasma chamber, again, a magnetic field segment MA1E of the firsttype, having the described effect on the ionization electron acceleratedin the direction of the intermediate electrode by the cathode EQ, ispresent between the beam exit SA and the intermediate electrode EZ2. InFIG. 2, in contrast to FIG. 1, the plasma chamber is assumed to have aconventional embodiment, in ring shape about a longitudinal axis LAT.The magnet arrangement then contains inner and outer magnet rings MRIand MRA, which lie opposite one another radially and have the samepoling. However, the generation of the primary electrodes is independentof the circular or ring-shaped chamber geometry and, in particular, theexternal cathode EQ is suitable as an electron source for bothgeometries.

Another possibility for the generation of ionization electrons in thelast potential stage is shown in FIG. 3. Here, the end electrode EEB isexposed to the bombardment and/or field influence of ions from an edgeregion RP of the plasma beam. Ions that impact the end electrode releaseelectron showers, for example, which are partly accelerated towards theintermediate electrode EZ2, as ionization electrons, and partly are alsocarried along by the plasma beam, as a neutralization electron streamNE. It is advantageous if the end electrode EEB consists of materialresistant to the ion bombardment, having a high secondary electronemission coefficient. Again, the magnetic field segment MA1E is providedbetween the end electrode EE4 and the intermediate electrode EZ2, butthe field progression is not explicitly shown in this drawing. Thepassive electrode is also particularly advantageous in combination withintermediate electrodes at sliding potentials.

The characteristics indicated above and in the claims, as well asevident from the drawings, can be advantageously implemented bothindividually and in various combinations. The invention is notrestricted to the exemplary embodiments described, but rather can bemodified in many different ways, within the scope of the ability of aperson skilled in the art.

1. A plasma accelerator system comprising: (a) an anode; (b) an endelectrode; (c) a plasma chanter between said anode and said endelectrode, said end electrode being spaced apart from the anode in alongitudinal direction of the plasma chamber at an exit of a plasma beamfrom the plasma chamber; (d) at least one intermediate electrodearranged between the anode and the end electrode in the longitudinaldirection, said at least one intermediate electrode lying electricallyat intermediate potentials; and (e) a magnet arrangement generating amagnetic field in the plasma chamber, said magnetic field comprising afirst set of first magnetic field segments and a second set of secondmagnetic field segments, said first magnetic field segments runningpredominantly perpendicular to the longitudinal direction in an areathat comprises the end electrode and the intermediate electrode thatlies closest to said end electrode in the longitudinal direction, saidsecond magnetic field segments running parallel to the longitudinaldirection, each first magnetic field segment having an adjacent secondmagnetic field segment on each side of the first magnetic field segmentin the longitudinal direction; wherein a last potential differencebetween the end electrode and the intermediate electrode that liesclosest to said end electrode amounts to at least four times a firstpotential difference between the anode and the intermediate electrodethat lies closest to said anode.
 2. The system according to claim 1,wherein ionization electrons are passed to the plasma chamber from theside of the end electrode.
 3. The system according to claim 2, whereinan electron source is arranged on the side of a plasma beam exit,outside the plasma chamber.
 4. The system according to claim 2, whereinat the exit from the plasma chamber, part of the plasma beam is passedto the end electrode, and the end electrode releases ionizationelectrons when this happens.
 5. The system according to claim 1, whereina bundled, accelerated electron beam is passed to the plasma chamberfrom the side of the anode.
 6. The system according to claim 1, whereinone of the first magnetic field segments lies between the end electrodeand the intermediate electrode that lies closest to said end electrodein the longitudinal direction.
 7. The system according to claim 1,wherein several first magnetic field segments follow one anotheralternately with second magnetic field segments in the longitudinaldirection.
 8. A plasma accelerator system comprising: (a) an anode; (b)an end electrode; (c) a plasma chamber between said anode and said endelectrode, said end electrode being spaced apart from the anode in alongitudinal direction of the plasma chamber at an exit of a plasma beamfrom the plasma chamber; (d) at least one intermediate electrodearranged between the anode and the end electrode in the longitudinaldirection, said at least one intermediate electrode lying electricallyat intermediate potentials; and (e) a magnet arrangement generating amagnetic field in the plasma chamber, said magnetic field comprising afirst set of first magnetic field segments and a second set of secondmagnetic field segments, said first magnetic field segments runningpredominantly perpendicular to the longitudinal direction in an areathat comprises the end electrode and the intermediate electrode thatlies closest to said end electrode in the longitudinal direction, saidsecond magnetic field segments running parallel to the longitudinaldirection, each first magnetic field segment having an adjacent secondmagnetic field segment on each side of the first magnetic field segmentin the longitudinal direction; wherein a last potential differencebetween the end electrode and the intermediate electrode that liesclosest to said end electrode amounts to at least four times a firstpotential difference between the anode and the intermediate electrodethat lies closest to said anode; and wherein several intermediateelectrodes are present with at least one intermediate potentialdifference between consecutive intermediate electrodes in thelongitudinal direction and the last potential difference amounts to atleast four times the greatest of the intermediate potential differencesor the first potential difference.
 9. A plasma accelerator systemcomprising: (a) an anode; (b) an end electrode; (c) a plasma chamberbetween said anode and said end electrode, said end electrode beingspaced apart from the anode in a longitudinal direction of the plasmachamber at an exit of a plasma beam from the plasma chamber; (d) atleast one intermediate electrode arranged between the anode and the endelectrode in the longitudinal direction, said at least one intermediateelectrode lying electrically at intermediate potentials; and (e) amagnet arrangement generating a magnetic field in the plasma chamber,said magnetic field comprising a first set of first magnetic fieldsegments and a second set of second magnetic field segments, said firstmagnetic field segments running predominantly perpendicular to thelongitudinal direction in an area that comprises the end electrode andthe intermediate electrode that lies closest to said end electrode inthe longitudinal direction, said second magnetic field segments runningparallel to the longitudinal direction, each first magnetic fieldsegment having an adjacent second magnetic field segment on each side ofthe first magnetic field segment in the longitudinal direction; whereina last potential difference between the end electrode and theintermediate electrode that lies closest to said end electrode amountsto at least four times a first potential difference between the anodeand the intermediate electrode that lies closest to said anode; andwherein the sum of the first and at least one intermediate potentialdifferences not including the last potential difference is not greaterthan the last potential difference.
 10. The system according to claim 9,wherein the sum of the first and at least one intermediate potentialdifferences not including the last potential difference is not greaterthan 50% of the last potential difference.
 11. The system according toclaim 10, wherein the sum of the first and at least one intermediatepotential differences not including the last potential difference is notgreater than 25% of the last potential difference.